1. Field of the Invention
The present invention relates to a semiconductor laser driving unit that controls driving of a semiconductor laser used for optical writing of laser printers, optical data communications, and optical disks, and to an image forming apparatus using the semiconductor laser driving unit.
2. Description of the Related Art
Generally, laser oscillation of a laser diode starts later than the start of current injection. The delay time is a time required to store carriers until the carrier density reaches the oscillation threshold of the laser diode after the start of injection of current into the laser diode. In order to avoid such oscillation delay, generally, a bias current is caused to constantly flow into the laser diode in the conventional semiconductor laser driving unit.
There are a fixed bias method and a variable bias method as methods of thus causing bias current to flow. According to the fixed bias method, a fixed bias current is caused to constantly flow into a laser diode without following variations in the oscillation threshold current (hereinafter, threshold current) of the laser diode or variations in the characteristics of the laser diode due to changes over time. Therefore, the difference in value between a determined bias current and the threshold current of the laser diode changes over time or differs depending on the individual laser diode, thus resulting in unstable oscillation delay.
According to the variable bias method, means for detecting the threshold current of an individual laser diode is provided, and a current approximating the detected threshold current is selected as a bias current. By intermittently repeating detection of the threshold current of the laser diode, it is possible to follow chronological variations or variations due to temperature changes in the threshold current. The variable bias method is suitable for avoiding variations in oscillation delay in driving a laser diode.
On the other hand, in semiconductor laser driving units, APC (Automatic Power Control) is a common practice as control means for constantly driving a laser diode that is a semiconductor laser with a predetermined amount of light emission. By performing APC, it is possible to obtain a laser driving current iop to flow into the laser diode, the laser driving current iop corresponding to a desired amount of light emission.
According to one semiconductor laser driving unit of a variable bias-fixed light emission current method, first, APC is performed so as to detect a light emission current iη corresponding to the difference between the threshold current ith of a laser diode and the current value of a laser driving current iop at which a desired amount of light emission is obtained. The light emission current iη of the detected value is generated as a fixed current in a digital-to-analog (D/A) converter (hereinafter, DAC) of a current output type, and the generated light emission current iη is caused to serve as a switching current to be controlled and supplied to the laser diode in accordance with an external modulation data signal. Further, every time APC is performed thereafter, a bias current up to the vicinity of the threshold current ith is determined.
Here, the current output-type DAC has a corresponding number of current cells for each bit. For example, as shown in FIG. 1, each bit is weighted with the number of current cells so that the number of current cells is 20 for the first bit, 21 for the second bit, 22 for the third bit, and 2n−1 for the nth bit. When a bit is “1,” current is output from one or more current cells corresponding to the bit. That is, when the nth bit is “1,” current is output from the corresponding 2n−1 current cells. Further, the current cells have the same shape and characteristics.
For this configuration, the speed of starting and stopping current output from each current cell with respect to an input code is always constant in the DAC. Therefore, it is possible to always supply the switching current to the laser diode at the same speed irrespective of the amount of light emission.
In the case of constantly causing a bias current around the threshold current ith to flow into the laser diode in this variable bias method, the amount of light emission by the bias current cannot be ignored particularly if the switching current is small. Generally, the intensity of light emission by a bias current in a laser diode may reach up to 300 μW. That is, if the intensity of light emission by the switching current is 1 mW, the amount of light of 30% of the intensity of light emission is attributed to the bias current. This weak light emission of a laser diode by a bias current causes problems such as background contamination in laser printers and digital copiers. If the inclination η (differential quantum efficiency) with respect to the amount of light emission of a laser diode produced by the light emission current iη is degraded and decreased by a change in temperature so that the light emission current iη increases, the above-described variable bias-fixed light emission current method supplements an increase in the laser driving current iop with a bias current. Therefore, the laser diode may emit light even in the logic of turning OFF the laser diode.
According to Japanese Laid-Open Patent Application No. 2004-153118, the above-described problem of the light emission of a laser diode even in the “OFF” logic is solved by providing the switching current with an offset current. Addition of an offset current to the switching current causes the bias current to be reduced by the amount of the offset current by APC. That is, the above-described problems can be reduced by determining the offset current so as to prevent the bias current from causing natural light emission and weak light emission at the time of turning OFF a laser diode due to a change in temperature, and by adding the bias current to the switching current.
However, according to the method of adding the offset current to the output current of the DAC, a circuit for generating the offset current should be newly added, thus causing an increase in the power consumption and the circuit area of a semiconductor laser driving unit.
Further, at the time of switching the laser diode from the “OFF” logic to the “ON” logic (the logic of turning ON the laser diode), switching means for performing output control of the offset current and switching means for performing output control of the switching current should be switched at exactly the same time. Further, the switching speeds of both switching means should be exactly the same, and the offset current and the switching current should have exactly the same rise time and fall time.
For example, if the semiconductor laser driving unit is formed of an IC, the differences in rise time and fall time between the offset current and the output current of the DAC due to process variations may cause the amount of light emission from the laser diode to have a step-like waveform, thus causing a problem in that it is difficult to obtain an intended light emission waveform of the laser diode. Further, there is a problem in that as the switching speed of each switching means increases, it becomes difficult to match the rise/fall timings of the output currents of different current generation parts.